If "vowed" means "promised solemnly":
* Broke a promise: This directly opposes the idea of keeping a vow.
* Reneged: This implies going back on a promise, often dishonorably.
* Recanted: This implies withdrawing a vow or statement publicly.
* Denied: This implies refusing to acknowledge or accept a vow.
If "vowed" means "declared a solemn commitment":
* Abandoned: This suggests giving up on a commitment.
* Disavowed: This implies rejecting or renouncing a previous commitment.
* Revoked: This suggests formally withdrawing a commitment.
If "vowed" means "sworn to do something":
* Refused: This simply means to decline to do something.
* Declined: This is a more polite way of saying "refused."
* Hesitated: This suggests being unsure or reluctant to do something.
The best antonym will depend on the specific context of how "vowed" is being used.